1. Field of the Invention
The subject of the invention is a single-blade impeller for centrifugal pumps which convey liquids mixed with solid additions, particularly which convey dirty water with long fibrous components. A channel is formed between the blade, whose thickness varies along its length, and the front and rear cover plates of the single-blade impeller.
2. Description of the Related Art
The designer of a single-blade impeller to be used mainly for conveying waste water is faced with a complex problem. He must not only design the single-blade impeller such that an accumulation of solid components in the conveying path, and especially a blockage caused thereby, is reliably avoided. He must also think of satisfying the somewhat conflicting requirement for a most favorable mass distribution of the single-blade impeller which, of necessity, is constructed unsymmetrically. And not last, he must strive for a large energy conversion in the single-blade impeller developed by him.
The fulfillment of the named requirements is affected by various problems which result from the conveying conditions and the characteristics of the single-blade impeller and can mutually influence and interfere with one another in many unforeseeable ways.
In most applications, centrifugal pumps with single-blade impellers are operated at constant rpm. However, the conveying conditions of such a centrifugal pump normally undergo continuous change. Thus, the positive pressure head, for example, varies between a maximum and a minimum value. The working point of the centrifugal pump, that is, the intersection of the constant pump characteristic line and the variable system characteristic line, accordingly is likewise subjected to the change.
Now, a change in the relative blade inflow speed, as well as a change in the relative inflow direction, are associated with the shift in the working point. This causes a static pressure distribution, which is specific to the particular blade shape, along the blade contour. Integration of the pressure distribution from the blade inlet edge to the blade outlet edge yields a blade force which rotates as a transverse deflecting force relative to a fixed reference point. This transverse force, which is also referred to as hydrodynamic upthrust, constitutes an harmonically operating exciting force which, in a system susceptible to vibration, that is, e.g., an installation, can cause big problems. It should, therefore, be kept as small as possible.
Another problem with the single-blade or single-channel impellets is constituted by the mechanical imbalance resulting from the unequal mass distribution of the impeller. It was now obvious to use the mechanical imbalance in compensating for the hydrodynamic upthrust. The resultant force obtained by vector addition of the two forces was equalized by an added weight. Now, however, the mechanical imbalance and the centrifugal forces are load-independent forces while the hydrodynamic upthrust also changes the angle of attack relative to the blade with the quantity being conveyed. Consequently, compensation of the mechanical imbalance and the hydrodynamic force can be achieved for only one working point of the centrifugal pump.
Hence, there is the requirement to maintain the mechanical imbalance of the single-blade impeller, in addition to the hydrodynamic upthrust, as small as possible. A geometric shape, whose hydrodynamic upthrust does not experience a change in direction within the working range, is to be sought for the blade. For an airplane wing, one would here speak of a profile with a fixed pressure point.
Another important aspect of the blade design is the optimization of the cavitation behavior of the centrifugal pump. Due to the hydrodynamic asymmetry caused by the system, stationary and unstationary exciting forces, which occur in the form of vibrations and endanger the operation of the centrifugal pump, can be released by single-blade impellets which undergo premature cavitation.
Various contributions, which concern themselves in depth with mass equalization and the danger of blockages, are known from the literature. However, less attention has here been given to the mutual influence of the various forces acting on the impeller. The influence of cavitation, in particular, has not been assigned the significance which it deserves in reality.
Thus, the U.S. Pat. No. 1,754,992 concerns itself with the problem of equalizing the mass imbalance in a single-blade or single-channel impeller. The cited patent provides two measures for overcoming this problem: First, the blade forming the channel has a thickness which decreases steadily along its length so that, in spite of the asymmetrical mass distribution, a mass imbalance is produced which is relatively small overall. Second, a counterweight serving for mass equalization is arranged on the rear side of the impeller. In addition, the provision of recesses on the rear side of the single-blade impeller is set forth as a third possible measure. The location of application can be varied to optimize equalization of the imbalance.
Based on the determination that even the channel impellers particularly well-suited for the conveyance of solid additions further tend to be blocked under unfavorable circumstances, the art contemplated a solution to the problem. On the assumption that irregularities in the guide channel and non-uniform flow behavior caused thereby lead to accumulations and blockage, one attempted to achieve a most uniform guide channel.
The article "Pumpen regeln Wasserhaushalt" (VDI-Nachrichten, No. 25/23, June 1965) then proposed an impeller shape for waste water systems which, to avoid an acute danger of blockage, was provided with a smooth passage. It was further important for this impeller that the cross section of the intake be maintained over the entire length of the channel.
A similarly designed impeller, but in which the shape of the channel cross section changes along the path, has become known from the U.S. Pat. No. 4,575,312. The size of the cross section is, nevertheless, to remain the same or to increase slightly towards the outlet. Since the guide channel, whose cross section is similar to a pipe elbow, must be purchased with a very thick blade, the U.S. Pat. No. 4,575,312 foresaw the possibility of making the blade partly hollow. This, however, does not yet eliminate the problem of impeller imbalance which cannot be compensated for completely and over an entire range of rpm. In particular, the still relatively great mass of even a partially hollowed out blade plays a significant role here. Moreover, such an impeller requires an increased cost during manufacture.